1. Field of the Invention
The present invention relates to a flow velocity detector which detects the flow velocity of a fluid including, for example, liquids, gases, liquid powders (powders), and aerosols and an ink-jet printer which is provided with the flow velocity detector.
2. Description of the Related Art
Conventionally, a flow rate meter (flow velocity meter), which is based on the Pitot tube system, is known as the flow rate meter (flow velocity meter) which measures the flow velocity of a fluid. As an example thereof, a flow rate meter based on the Pitot tube is known, the flow rate meter including a Pitot tube which is arranged in a gas flow of a gas to be measured, a differential pressure detector which detects the differential pressure between the total pressure in a total pressure-detecting tube of the Pitot tube and the static pressure in a static pressure-detecting tube, and a calculating means which calculates the flow velocity of the gas in accordance with a detection output of the differential pressure detector (see Japanese Patent Application Laid-open No. 2004-294147).
An ultrasonic flow rate meter is known as another example of the flow rate meter. As an example thereof, an ultrasonic flow rate meter is known, wherein an ultrasonic wave signal, which is generated by an ultrasonic vibrator disposed on the upstream side of a tube passage, is received by an ultrasonic vibrator disposed on the downstream side, and an ultrasonic wave signal, which is generated by the ultrasonic vibrator disposed on the downstream side, is received by the ultrasonic vibrator disposed on the upstream side. Two pieces of digital data, which are obtained by sampling the signals respectively, are determined, and then the total sum of differences between two signal arrays is determined in a calculating unit while deviating the time axes of the pieces of data. The transmission time difference is determined in CPU on the basis of the point at which the total sum of the differences is minimized to determine the flow velocity of the fluid (see Japanese Patent Application Laid-open No. 2007-298347).
An ultrasonic Doppler flow rate meter is known as still another example of the flow velocity meter. As an example thereof, an ultrasonic Doppler flow rate meter is known, which measures the flow rate of the fluid allowed to naturally flow downwardly in a tube passage in accordance with the following procedure. At first, an ultrasonic wave having a constant frequency is transmitted at an angle of elevation in the flow direction from a transmitting element provided at a central portion of the bottom of the tube passage. An emission wave is received by a receiving element to obtain a beat waveform including the information of a Doppler shift frequency by means of a heterodyne detecting section. Subsequently, the signal except for the noise is allowed to pass by means of a bandpass filter, and the AD conversion is performed by means of an AD converter. After that, a frequency spectrum of the beat waveform is determined by means of a high speed Fourier transformation section. Further, a peak frequency in the frequency spectrum (Doppler shift frequency) is determined by means of a peak detecting section. The average flow velocity and the flow rate are calculated on the basis of the peak frequency by means of a flow rate calculating section. In accordance with the procedure as described above, it is possible to measure the flow rate of the fluid allowed to naturally flow downwardly in the tube passage (see Japanese Patent Application Laid-open No. 9-229734).
However, in the case of the conventional flow rate meter based on the Pitot tube, the ultrasonic flow rate meter, and the ultrasonic Doppler flow rate meter, the structure is complicated and large-sized and the cost is expensive as well. Therefore, it is desired to use a small-sized and low cost flow rate meter, for example, when the flow velocity is measured for the ink allowed to flow through an ink flow passage of an ink-jet printer. However, it is difficult to use the conventional flow rate meter, for example, for the way of use to measure the flow velocity of the ink in an ink-jet printer, because the structure is complicated and large-sized and the cost is expensive as well in the case of the conventional flow rate meter as described above.